An optical system which uses a small-type image pick-up lens element having an outer diameter of several millimeters or less has primarily been used in the prior art as an image pick-up optical system of an endoscope. However, image pick-up optical systems have recently been used for various other applications as well. The image pick-up lens element of an optical system of an endoscope has been manufactured by the same process conventionally used for making the lens elements of photographic cameras and microscopes. That is, one surface of the lens element surface undergoes a rough grinding, followed by a precise grinding, and then a polishing. The other surface of the lens element is then processed similarly.
There has been a demand for further reduction in the size of image pick-up optical systems. Moreover, it does not appear that conventional manufacturing processes are well-suited for manufacturing lens elements of the size required. For example, in an image pick-up optical system of an endoscope, the trend has been to reduce the diameter of the endoscope as well as the length of the non-flexible end portion that is suitable for housing the image pick-up optical system Thus, a smaller diameter lens element having higher refractive power is required. However, accurately manufacturing and mounting such a lens element, as well as inspecting the lens element for accuracy of manufacture and mounting, becomes more difficult as the lens element is made smaller. When the lens element surfaces are processed in the conventional manner, as the size of the optical system is decreased the lens element construction becomes more difficult, the tolerance of the optical system for normal manufacturing and assembly variations decreases, and the number of inspections needed to assure high quality increases.
Meanwhile, there have been recent improvements in the technology of using a mold to simultaneously form opposite surfaces of a lens element by pressing the optical material into the desired shape. Such a molding technique has mainly been used to form aspherical lens elements used in photographic cameras. Of course, this requires that both dies be manufactured with high precision and accuracy, as a die's imperfection(s) will be transferred to each lens element surface produced by the die. However, once a die set of high accuracy and precision has been prepared and inspected, there is little need to inspect each lens element produced by the die set. Therefore, if a molding technique can be applied to the manufacturing of a lens element having a small outer diameter and strong surface curvature, the problem of an excessive number of inspections being required in order to assure high quality of such a lens element will be decreased.
In the usual molding technique for manufacturing a lens element for a photographic camera, a preform material having a shape that approximates that of the final lens element is first molded with a die set by pressing. For this reason, the amount of displacement in the final press molding of a camera lens element is very small. However, when molding very small lens elements such as used in endoscopes, it is necessary to mold the lens element in a one-step process from a parallel plate material in order to keep the cost per lens element produced from becoming excessive. Thus, the amount of displacement of the moveable die of the die set in pressing such a lens element is inevitably large. This requires that the lens element material be heated to a temperature above the glass transition point, and that the pressing occurs in such a state.
A glass material S-BSL7, made by the Ohara Corporation, having a glass transition point of 565 degrees Centigrade has been unsuccessfully tested for molding very small diameter lens elements having strong surface curvatures, as used in endoscopes. Because the molding temperature must be high, as discussed above, and because the diameter of the lens is very small, a large pressure must be applied to the molding surface of the die. For this reason, part of a protective film formed on the molding surface to protect the die peals. To overcome this difficultly a glass material SF11 (made by Schott Glass), which has a glass transition point of 503 degrees Centigrade and which contains lead, was tested. Although the protective film of the molding surface of the die does not peal in this instance, the lead in the glass tends to precipitate onto the surface of the die and to adhere to it during the molding process.
When either peeling of the film on the molding surface or precipitation of lead onto it occurs, a defect results on the surface of a lens that is press-formed by the die. For this reason, making a small diameter lens element having a strong surface curvature (as used, for example, in an optical image pick-up system of an endoscope) by the prior art molding technique has been impossible. In the case of a small-sized optical system apparatus as used for an image pick-up of an endoscope, the diameter of beam carrying image information is necessarily small as compared with an optical system apparatus of a usual photographic camera. Thus, even if the surface defect of the image pick-up lens is very small, the amount of degradation of image quality is large. This requires that the molding surface of each die be made with high precision and accuracy.